notes 5 hmr 3 coupling 2

7/21/2019 Notes 5 Hmr 3 Coupling 2

1/17

E s d t

s d

t q

dd

Two equal couplings.

s d dd

Two different couplings.

HC C

H HC C

H

X =

X =

X =

X =

X =

X =

X =

X =

A =

A =

JAX

> 0JAX

< 0

Coupling constants are positive if the spin state in which A and X have

opposite spins () is lower in energy than the one in which they have

the same spin ().

The signs of couplings shows some consistency.

1JC-Hand most other one-bond couplings are positive.

2JH-Hin sp3CH2groups are almost all negative, some others are positive.

3JH-His always positive.

d

X=

X=

d

X=

X=

A-signal

Sign of Coupling Constants

The actual local magnetic field encountered by a nucleus is affected by neighboring magneticnuclei (spin-spin

splitting) as a result of perturbation of the electron distribution. The principal magnetic nuclei are other protons, the

100% abundant spin nuclei 19F and ~31P, and some spin 1 or greater (quadrupolar) nuclei such as 14N, 2H, 11B,

and 12B. Although Br, Cl, and I all have isotopes with spin >, coupling is not seen because of relaxation effects.

This will be discussed in more detail in Section 7.

5.3 Spin-Spin Splitting J-Coupl ing

Coupling constants can be either positive or negative. These are defined as shown below:

For first order patterns the signs of the couplings have no effect on theappearance of the spectrum, and so cannot be determined by

observation. However, decoupling experiments (spin tickling) can

provide the relative signs. For second-order patterns (e.g. ABX or

AA'BB'), the relative signs of coupling constants often have dramatic

effects on the appearance of the spectrum, and relative signs can be

determined by proper analysis of the multiplets.

Note: vertical scale of the couplings is grossly exaggerated

Copyright Hans J. Reich 201

All Rights Reserved

University of Wisconsin

5-HMR-3.1


2/17

Two Different Couplings to one Proton

Consider the NMR spectrum of 3,4-dichlorobenzoyl chloride below.

9 8 7 6 5 4 3 2 1 0

ppm

Cl O

Cl

Cl

R-19M C7H3Cl3O

270 MHz 1H NMR Spectrum (CDCl3)

8.2 8.1 8.0 7.9 7.8 7.7 7.6 7.5

ppm

0Hz

10203040

The proton-proton couplings in benzene are typically 7-9 Hz for Jortho, 2-3 Hz for Jmetaand


3/17

A slightly more complicated case is 1,1,2-trichloropropane. A simulated spectrum is shown below.

The C-2 proton is coupled to one proton at C-1 and three protons of the methyl group at C-3. Naively, one might

expect a pentet(p), as shown in the left spectrum below. Although pentets are, in fact, often observed in such

situations, this occurs only if J1-2and J2-3are identical. When they are not (as is actually the case in this example),

then we get a quartet of doublets(qd). It is customary to quote the larger coupling first (q) and then the smaller

coupling (d). A proper text description of the multiplet is: 4.30, 1H, qd, J= 6.6, 3.8 Hz.

4.0

J12= 3.8 Hz

J23= 6.6 Hz

J12= 5.0 Hz

J23= 6.6 Hz

J12= 6.6 Hz

J23= 6.6 Hz

4.5

Cl C

Cl

H1

C

Cl

H2

CH3

Hypothetical Hypothetical Actual

4.04.5 4.04.5

6.6 Hz

3.8 Hz

Exercise: what would a dq, J= 6.6, 3.8 Hz look like?

8 7 6 5 4 3 2 1 0ppm

Chemical Shift (in ppm)

(1) = 5.850(2) = 4.300(3) = 1.620(4) = 1.620(5) = 1.620

Coupling constants (in Hz)2 3 4 5

J(1:y) 3.80 0.00 0.00 0.00J(2:y) 6.60 6.60 6.60J(3:y) 0.00 0.00J(4:y) 0.00

WINDNMR Simulated Spectrum of 1,1,2-Trichloropropane

H C

Cl

Cl

C

Cl

H

C

H

H

H

5-HMR-3.3


4/17

First Order Coupling Rules

1. Nuclei must be chemical shift nonequivalent to show obvious coupling to each other. Thus the protons of

CH2Cl2, Si(CH3)4, Cl-CH2-CH2-Cl, H2C=CH2and benzene are all singlets . Equivalent protons are still coupled to

each other, but the spectra do not show it.

2. Jcoupling is mutual, i.e. JAB= JBAalways. Thus there is never just one nucleus which shows Jsplitting -

there must be two, and they must have the same splitting constant J. However, both nuclei need not be protons -

fluorine (19F) and phosphorus (31P) are two other common nuclei that have spin and 100% abundance, so they

will couple to all nearby protons. If these nuclei are present in a molecule, there are likely to be splittings which arepresent in only one proton multiplet (i.e. not shared by two multiplets).

3. Two closely spaced lines can be either chemically shifted or coupled. It is not always possible to distinguish J

from by the appearance of the spectrum (see Item 4 below). For tough cases (e.g. two closely spaced singlets in

the methyl region) there are several posibilities:

decouple the spectrum

obtain it at a different field strength (measured in Hz, coupling constants are field independent, chemical shifts

are proportional to the magnetic field)

measure the spectrum a different solvent (chemical shifts are usually more solvent dependent than coupling

constants, benzene and chloroform are a good pair of solvents).

For multiplets with more than two lines, areas, intensities, symmetry of the pattern and spacing of the lines

generally make it easy to distinguish chemical shift from coupling.

For a simple example see the spectrum of 3-acetoxy-2-butanone below. Here it is pretty easy to identify one of

the doublets as the 4-methyl group, the other "doublet" (with a separation of 9 Hz, which could easily be a coupling)

actually corresponds to the two CH3C(=O) groups.

5 4 3 2 1 0ppm

1.00

5.88

2.91

5.15 5.10 5.05 5.00

2.20 2.15 2.101.40

O

O

O

300 MHz 1H NMR in CDCl3Source: Aldrich Spectra Viewer/Reich

0Hz

102030

4. Chemical shifts are usually reported in (units: ppm) so that the numeric values will not depend on thespectrometer frequency (field-independent units), coupling constants are always reported in Hz(cycles per

second). Chemical shifts are causedby the magnetic field, couplings are field-independent, the coupling is

inherent in the magnetic properties of the molecule. However, all calculations on NMR spectra are done using Hz

(or, more precisely, in radians per sec).

5-HMR-3.4


5/17

H

Hsmall

0-3 Hz

6. Multiplicity for first order patterns follows the "doubling rule". If all couplings to a particular proton are the

samethere will be 2nI+1 lines, where I is the spin and n is the number of neighboring nuclei (n + 1 for 1H I = 1/2).

The intensities will follow Pascal's triangle.

11 1

21 13 3 11

1 4 6 4 11 5 10 10 5 1

1 6 15 20 15 6 1

triplet n = 2 quartet n = 3 pentet n = 4 sextet n = 5 nonet n = 8Intensities

7. If all couplings are different, then the number of peaks is 2nfor 1H, and the intensities are 1:1:1: . . .. Thus a

proton coupled to two others by different couplings gives a dd (doublet of doublets, see Figure). This pattern is

nevercalled a quartet. As the number of couplings gets larger, accidental superpositions of lines will sometimes

occur, so that the 1:1:1... intensity ratio no longer applies. The intensities are also often distorted by leaning effects,

as seen in several examples below.

dd n = 2 ddd n = 3 ddd n = 3 dddd n = 4 ddddd n = 5

8. More typically, some of the couplings are the same, others different, so get a variety of patterns. In

favorable cases, these patterns can be analyzed and all couplings extracted. The number and size of couplings

(J-values) provide important structural information.

C

H

H

2J= 2-15 Hz

HC

CH

3J= 2-20 HzHC

C

C H

4J= 0-3 Hz

geminal vicinal long-range

Typical: -12 Hz Typical: 7 HzOccasionally 0!

Usually 0

5. Protons two (2J, geminal) or three bonds (3J, vicinal) apart are usually coupled to each other, more remote

protons (4J, 5J) may be if geometry is right, or if -systems (multiple bonds) intervene. Long range couplings (4Jor

greater) are usually small, typically


6/17

Second Order Effects

1. Protons or groups of protons form simple multiplets only if the chemical shift differences between the protons

() are large compared to the coupling constants between them (J). If /J(all in Hz) is


7/17

(c) Coupling constants and chemical shifts can no longer be easily determined from the spectrum. The sign of the

coupling constant is significant. "Virtual coupling" effects (apparent coupling to protons that are actually not coupled)

may appear (see Section 5.16).

2. For a given molecule /Jincreases as the magnetic field increases, hence spectra are usually simpler at

higher field (better dispersion).

3. Second order effects will appear even if /Jis large when groups of magnetically non-equivalentprotons

with identical chemical shifts are coupled to each other (see Section 5.8). Thus Me3Si-CH2-CH2-Cl (an AA'XX'system, see Section 5.15) is not just two triplets. These patterns do not get simpler at higher field strengths.

These effects will be dealt with in some detail in subsequent sections on the various coupling patterns. 01/39

5-HMR-3.7


8/17

5.3.7 Rules for Analyzing First Order Multiplets

A first order multipletcan be expected when bothof the following criteria are met:

First, the chemical shift of the observed proton must be far away from any of the protons it is coupled to (far

away means >> J). In practice, multiplets can be treated in a first order fashion if > 3J, although the

substantial leaning distortions can complicate analysis. The leaning will have almost completely disappeared by th

time = 10J.

Second, if more than one proton is coupled to the observed one, then these protons must not be "strongly

coupled." In othere words, if they are coupled to each other and very close in chemical shift then the observed

proton multiplet will not yield true coupling constants on analysis, even though it looks first order. See the section o

Virtual Coupling.

Structure of First Order Multiplets. The fundamental rule governing multiplet intensities for spin 1/2 nuclei wit

all couplings identical is Pascal's triangle (n = number of equivalent couplings).

n 2n Multiplet Intensities

0 1

1 22 4

3 8

4 16

5 32

6 64

7 128

A first order multiplet consists of the product(not the sum) of several such multiplets. In other words, a single lin

will first be split into one of the symmetrical multiplets (1:1 d, 1:2:1 t, 1:3:3:1 q, etc), then each line of this multiplet

will be again split into d, t, q, or higher multiplet.

1

1 121 1

3 3 11

1 4 6 4 1

1 5 10 10 5 1

1 6 15 20 15 6 1

1 7 21 35 35 21 7 1

1 8 28 56 70 56 28 8 1Nonet

Octet

Septet

Sextet

Pentet

8 256

5-HMR-3.8


9/17

1. All truly first order multiplets are centrosymmetric- there is a mirror plane in the middle (in real spectra, this is

usually not strictly true because of leaningand other distortions). However, the reverse is not true: not all

symmetrical multiplets are first order.

2. If the small outermost peaks are assigned intensity 1, then all other peaks must be an integral multiple

intensity of this one (1x, 2x, 3x, 4x in height), and the total intensity of all peaks must be a power of 2 (2, 4, 8, 16, 3

etc). The intensity of each of the two outermost lines is 1/2nof the total multiplet intensity, where n is the number o

protons which are coupled with the proton signal being analyzed. There can be no lines smaller than the outermos

one. Note, however, that if n is large, the outermost peaks may not be distinguishable from noise. Intensity

assignments and determination of n cannot be easily made for such multiplets

Recognizing a First Order Multiplet.

1

2

11

2

1

3

6

33

6

3

1 1 1 1 1 1

2

1

2

11 1 1 1

3 3

2

= 8 = 16 = 32

Coupled to 3 other protons

23= 8


24= 16


25= 32

3. There is a strict regularity of spacing in a first order multiplet: if you have correctly identified a coupling

constant J, then everypeak in the multiplet must have a partner JHz away to the left or to the right of it.

1

2

1 1 1 1 1

3 3

2This separation is

always a true coupling

This is also a coupling

This is not a coupling

This is not a coupling

4. Most first order multiplets integrate to a single proton, a few may be 2 or 3 protons in area. It is rare to have

more than 3 protons, unless there is symmetry in the molecule (e.g., (CH3)2CH- gives a 6-proton doublet for the

methyl groups). Thus a 4-proton symmetrical multiplet is usually not a first-order pattern (it is more likely to be the

very common AA'BB' pattern).

5. The symmetry and intensities of an otherwise first-order multiplet can be distorted by leaningeffects (see

Section 5-HMR-9). Many such multiplets can still be correctly analyzed by first-order techniques, but you have to

mentally correct for the intensity distortions. However, the coupling constants extracted may not be perfectly

accurate.

5-HMR-3.9


10/17

Analyzing a Fi rs t Order Mul tiplet. First order multiplets are analyzed by constructing a reverse coupling tree,

"removing" each of the couplings in turn, starting with the smallest.

1. "Take out" the smallest couplings first. The separation between the two lines at the edge of the multiplet is

the smallest coupling. Each time you remove a coupling you generate a new, simpler multiplet, which can then be

analyzed in turn. Remember that each line of the multiplet participates in each coupling.

2. Watch line intensities (i.e., peak areas or peak heights) carefully--when you "take out" a coupling, the

intensities of the newly created lines should be appropriate (i.e., each time you "take out" a coupling, also "take ou

the proper intensity). When a coupling has been taken out completely, all intensity should be accounted for. Keep

track of your analysis by using a "coupling tree".

3. The couplings may be removed one at a time as doublets, or as triplets, quartets and higher multiplets. The

intensity ratio of the first two lines signals the number of protons involved in the coupling: 1:1 means there is only

one proton, 1:2 means that there is a triplet splitting (2 protons), etc. Be especially careful to keep track of intensit

when you "take out" triplets (1:2:1) or quartets (1:3:3:1). Each time you completely remove a coupling you genera

a new multiplet which follows first order rules, and can be analyzed in turn.

When you have finished your analysis, all peaks in the multiplet must be accounted for. You can check the

analysis as follows: the separation of the two outermost peaks of the multiplet is the sum of all the J's (i.e., for a dt

= 8, 3 Hz the outermost lines are separated by 8 + 3 + 3 = 14 Hz).

Reporting a First Order Multiplet . Multiplets are reported starting with the largest coupling, and the symbolsmust be in the order of the reported numbers: 2.10, 1H, qt, J= 10, 6 Hz means: a single proton q of 10 Hz, t of 6

with a chemical shifts of 2.10 ppm.

Quartets. Keep clear in your mind the distinction between a simple q (one proton equally coupled to 3 others,

with an intensity 1:3:3:1), an ABq (2 protons coupled to each other, see Sect 5-HMR-9), and the quartet formed by

coupling with a spin 3/2 nucleus (e.g., 7Li, intensity 1:1:1:1, see Sect 7-MULTI-2.4). Only the first of these should b

referred to by just a "q" symbol. The early NMR literature (and even modern novices) sometimes call doublets of

doublets "quartets" (there are four lines, after all).

AB quartetquartet 1:1:1:1 quartet doublet of doublets

5-HMR-3.10


11/17

50 40 30 20 10 0

Practice Multiplets Chem 605Reich

Hz

5-HMR-3.11


12/17

The separation between the first and second lines is the smallest

coupling in the multiplet (J1). They are in an intensity ratio of 1:1, sothat coupling appears only once (i.e., it is a doubletsplitting).

"Remove" the first coupling. Keep track of line intensities, and draw

a child multiplet with positions at the center of each doublet. Each

unit of line intensity can only be used once, and all line intensity

must be accounted for

Repeat the process - the first and second lines now represent the

next largest coupling. Remove this in turn, and repeat the process

until you get a singlet

J1

J1

J2

J3

Here again the separation of the first two lines is the smallestcoupling in the multiplet (J1). However, they are in an intensity ratio

of 1:2, so this coupling appears twice (i.e., it is a tripletsplitting).

J2

J3

1

2

1 1 1 1 1

3 3

2

"Remove" the triplet coupling. Keep track of line intensities, and

draw a child multiplet with positions at the center of each triplet.

Note that the central two lines of intensity 3 are the sum of two lines

- one of intensity 1 and the other intensity 2.

This is a ddt, J= 16, 10, 6 Hz

This is ddd, J= 12, 10, 4 Hz

First Order Analysis

1 1 1 1 1 1 1 1

= 8 (3 couplings present)

= 16 (4 couplings present)

5-HMR-3.12


13/17

Note the leaning, indicating that the coupled partner is close by.

Doublet of doublet of doublets (ddd)

Doublet of doublets (dd) Triplet of doublets or doublet of triplets (dt, td)

Doublet of quartets (dq)The multiplet below integrates to two protons.

Using the chemical shift (5.8) and coupling

identify the structural fragment.

Simple Multiplets

3.05 3.00

ppm

2.45 2.40

ppm

O

O

Ph

7.05 7.00 6.95 6.90

ppm5.90 5.85 5.80

ppm

OMe

O

OMe

O

3.00 2.95

ppm

2.55 2.50 2.45

ppm

O

O

O

5.85 5.80ppm

O

O

O

5.95 5.90 5.85 5.80 5.75 5.70 5.6

ppm

2.70 2.65 2.60

ppm

H

O

Cl

O

Assign the protons where structure and chemical shift scale are given

0Hz

10203040

5-HMR-3.13


14/17

Symmetrical Mult iplets which are NOT First Order

Only ONE of the multiplets below is first order

0Hz

1020304050

Some criteria to use:

Pattern must be centrosymmetric (true of all of these)

Intensity of lines - patterns must be repeated, especially examine outer lines

Be wary if #H > 1, especially if 4H

Consider size of possible couplings

(a) (b)

(c) (d)

(e) (f)

5-HMR-3.14


15/17

5-HMR-3.15 Measurement of Coupl ing Constants

The accurate measurement of Jcoupling constants requires that the multiplets be correctly analyzed. In the

following pages are described techniques for performing such analyses. The procedures are summarized below.

For first order multiplets a simple "tree" analysis as described in Section 5-HMR-3.9 can directly yield coupling

constants within the accuracy of the digital resolution of the spectrum. This includes AB spectra, where JABcan be

measured directly. See section 5-HMR-7 for a description of the ABC.. nomenclature for spin systems.For AB2spectra both the coupling constant JABand the chemical shifts can be obtained by simple arithmetic

manipulations, provided that line assignments can be made correctly. For ABX spectra JABis accurately

measureable by inspection. An approximate analysis, which treats the peaks as AMX, will give values for JAXand

JBXthat will be in error by varying amounts, depending on the relative size of AB, the relative size of JAXand JBXand the size of JAB(the smaller ABthe larger the error). To get accurate values for the JAXand JBXcoupling

constants a proper ABX analysis as described in Section 5-HMR-12 is required.

For many simple compounds the symmetry is such that protons are homotopic or enantiotopic, and no coupling

constants can be measured directly (e.g., the 2Jcoupling in methane or dichloromethane; the ortho, meta, and para

couplings in benzene; the cis, trans and gem couplings in ethylene, etc). For such compounds the following

techniques are used to measure JHH:

Isotopic Substitution. Replacing one of the protons by deuterium (or even tritium) breaks the symmetry of the

coupled system and allows measurement of JHD(or JHT). The value of JHHcan then be calculated from the

gyromagnetic ratios. In the example below, the 60 MHz NMR spectrum of a mixture of undeuterated (s),

monodeuterated (1:1:1 triplet, the spin of D is 1, see Sect. 7-MULTI-2.1) and dideuterated (1:2:3:2:1 quintet)

acetonitrile is shown. Note the isotopic shifts. (Grant, D. M.; Barfield, M. JACS 1961, 83, 4727)

120 115Hz

CD2HCN

CDH2CN

CH3CNJHH

JHD=

HD

= 6.488

JHH = 6.488 JHD

JHD(CH2D-CN) = 2.58 Hz

JHH(CH3-CN) = 16.75 Hz

Analysis of Complex Spin Systems . In molecules where the chemical shift-equivalent protons are of the AA'type (part of an AA'XX', AA'X3X3' or similar system), complete analysis of the spectrum sytem can, in favorable

circumstances, give the value of JAA'. An example is 1,3-butadiene, an AA'BB'CC' system in which all protons are

compled to all other ones. Analysis of the complex NMR spectrum gave, among numerous other couplings, values

for the following couplings between chemical shift equivalent nuclei: 3JAA',5JBB'and

5JCC'(Hobgood, R. T., Jr.;

Goldstein, J. H. J. Mol. Spectr.1964, 12, 76).

HA'

HA

HC

HBHB'

HC'

5.4 5.2 5.0 46.6 6.4 6.2 6.0

A BC

60 MHz

Solv: cyclohexane

3JAA'= 10.415JBB'= 1.305JCC'= 0.69

3JAB= 10.174JAB'= -0.863JAC= 17.05

4JAC'= -0.832JBC= 1.745JBC'= 0.60

5-HMR-3.15


16/17

8 7 6 5 4 3 2 1 0ppm

1300 1250 1200 1150

HOMe

O

HOMe

O

3JHH

1JHC

Spinning

side bands

5.2 5.0 4.8 4.6 4.0 3.8 3.6

9 8 7 6 5 4 3 2 1 0

ppm

2.00

3.97

OO

CDCl3, 300 MHz

W. S. Goldenberg

CDCl3200 MHz

1JHC AA'BB'X

C3H6O3

C6H8O4

Analysis of 13C Satellite Spectra. Vicinal couplings between homotopic or enantiotopic protons 3JHHcan often

be obtained by analysis of the 13C satellites. The 1H NMR signal for the vinyl protons of dimethyl maleate is a

singlet. However, the 13C satellites are doublets, with a splitting that is equal to 3JHH. In effect, the A2spin system of

the 12C isotopomer has become an ABX pattern in the mono-13C labelled compound, where X is the 13C nucleus,

and A and B are the two vinyl protons, one on 13C and the other on 12C.

For systems of the X-CH2-CH2-X type, the mono-13C isotopomer is an AA'BB'X pattern, which can be solved to

obtain JAA'(= JBB') as well as JABand JAB'.

1JHC

X

13C Satellite13C Satellite

13CH

OMe

O

H

12C OMe

O

O

13C

O

H H

OO

13C 12C

HA'

HAHB'

HB

The BB' protons

are hidden under

the central peakO

12CO

H H

2.2% of the molecules have 13C

at one of the vinyl carbons

5-HMR-3.16


17/17

8 7 6 5 4 3 2 1 0ppm

Ph

Masamune, S. J. Am. Chem. Soc.1964, 86, 735

2.88 (190)

4J= 14 Hz

Ph

HH

O

4JHH= 14 Hz

1JHC= 190.1 Hz

Below is an example of the measurement of a 4JHHin a symmetric tricyclic system using the13C satellite method

notes 5 hmr 3 coupling 2

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